The invention relates a method for calibrating an electronically phase-controlled group antenna, using a reference point shared by all the reference signals, in radio communications systems and to an arrangement for this.
By the use of electronically phase-controlled group antennas, known as intelligent antennas, in radio communications systems, such as for example digital mobile radio systems, a directional selectivity of a mobile radio channel that exists in spite of multipath propagation can be advantageously used for the radio communication.
Intelligent antennas form a radiation pattern by corresponding phase-directed activation of the individual antenna elements of the antenna array. The beam forming can therefore be used to transmit a message from a base station to a subscriber station specifically in the direction of the latter. As a result, on the one hand the sensitivity to interferences in the particular radio cell of the base station can be reduced and on the other hand co-channel interferences in neighboring radio cells can be reduced. Moreover, the range of a base station which is providing a specific mobile station with radio resources increases significantly for the same transmit power. In addition, as a consequence of the spatial separation, physical channels within a radio cell served by a base station can be reused and the antenna lobes, as they are known, of the directional diagram can be adaptively corrected when subscriber stations move.
To achieve a desired beam formation, the original transmission signal is sent via a plurality of antenna elements, usually with different, but defined phase angles. The corresponding phase angle is ascertained for each antenna element by a digital signal processing (DSP).
Unforeseeable phase errors and time delays generally occur when setting the phase angle in the analog area between digital/analog converters and antenna elements. As a result, the transmission signals are not sent with the desired phase angles and the beam formation is falsified or even impossible. To counteract this unfavorable property of the analog area of beam formation, what is known as antenna calibration is necessary. Antenna calibration eliminates the influence of the entire analog signal chain on the errors described above.
To use beam formation, firstly the direction of the base station in relation to the mobile station must be established. The direction is established by evaluation of the various phase angles of the received signal at each antenna element of the antenna array. Therefore, an antenna calibration in the base station is necessary not only for the downlink to the subscriber station but also for the uplink from the subscriber station to the base station.
In a TD-SCDMA system (Time Division-Synchronous Code Division Multiple Access System), using intelligent antennas, an additional antenna, known as a reference antenna, is used for the antenna calibration. For the case of an uplink calibration, a reference signal is sent via the reference antenna to all the antenna elements of the antenna array. At the individual antenna elements, a specific delay time and a specific phase position, depending on the distance from the reference antenna, are expected on account of the finite propagation velocity of electromagnetic waves. The difference between the expected setpoint value and the actually measured actual value is ascertained and stored as a correction factor. The correction factor is then included in the normal signal processing process, whereby the antenna is calibrated.
For the downlink calibration, the reference antenna receives at a specific point in time a reference signal from an antenna element of the antenna array, and the correction factor is determined. To counteract the distortion of the measurement results on account of different antenna elements of the antenna array, they must not transmit a signal at this point in time. Subsequently, at a second point in time, the reference antenna receives a reference signal from a second antenna element of the antenna array, and the correction factor for this second antenna element is determined, and so on. For the calibration of n antenna elements of the antenna array, accordingly n time slots must be used when supporting a TDMA subscriber separation method (Time Division Multiple Access).
The error in the delay time is often only a fraction of a chip (chip=CDMA code element). To take such a small delay time into account in the signal processing, an oversampling of the received signal and transmission signal is necessary. However, oversampling makes the data rates to be transmitted considerably greater.
The invention is based on the object of significantly shortening the time for the calibration of intelligent antennas in the downlink.
A further object is to perform a correction of the analog error without the necessity of calculating a correction factor for each antenna element and without oversampling and the associated higher data rates.
A further object is to keep down the load on the transmission capacity of physical channels caused by an antenna calibration.
According to one aspect of the invention, all the antenna elements of an intelligent antenna in the downlink are calibrated in only one step. For this purpose, reference signals which can be distinguished from one another are simultaneously sent by the individual antenna elements of the antenna array and are separated again after reception at a reference point shared by all the reference signals.
An advantageous refinement provides a separation of the reference signals using a CDMA method (CDMA=Code Division Multiple Access), which is based on a separation of signals by individual spread-spectrum codes.
In a further refinement, conventional spread-spectrum code techniques, such as correlation, in which the common reference point is synchronized to the respective reference code channel of the antenna elements and the reference signals are again reduced to their original bandwidth, are used for the separation of the reference signals.
According to a further refinement, in this case the reference signals are orthogonally coded, in order that the interferences remain minimal in spite of simultaneous transmission.
The calibration factor can be obtained from the result of the correlation in a digital signal processor.
Another advantageous form of the invention is to use an optimized amount of reference signals, which allows an impartial estimate of the calibration factor.
The generation of such an optimized amount of reference signals and of the estimated value can be performed in an advantageous way by methods which are described in: Bernd Steiner, Paul Walter Baier: xe2x80x9cLow Cost channel Estimation in the uplink receiver of CDMA mobile radio systemsxe2x80x9d, Frequenz 47 (1983), pages 292-298.
According to a further form, the correction of the delay time, phase error and/or amplitude of the transmission signals can be performed directly within a digital UP-conversion/down-conversion, whereby no correction factor has to be included and no oversampling of the received signal and transmission signal is necessary to eliminate delay errors.
For this purpose, tuning of the numerically controled oscillator (NCO) of the digital UP-converter (DUC) and of the digital down-converter (DDC) takes place.
In a further development, in a TDD system the calibration is carried out in the delay time without transmission between the uplink and downlink time slots.
In a further refinement, the downlink calibration may take place at the beginning of the delay time and the uplink calibration may take place at the end of the delay time.
In a further refinement, a reference antenna is used as the shared reference point for the reference signals from and to the antenna elements.